Recently, regarding optical scanning devices used for image formation apparatuses in electrophotographic systems, etc., resin optical elements such as long lenses and mirrors exemplified by a scanning lens and scanning mirror have been used. Using resin is advantageous in that light and inexpensive elements having a complex shape suitable for the optical scanning device side with respect to such aspects as size and layout can be relatively easily obtained.
On the other hand, a method of obtaining optical elements using molding technology referred to as injection molding is known as one resin manufacturing method of optical elements. For example, injection molding is carried out as follows. First, melted liquid resin is injected into a die via the nozzle of an injection molding machine. Resin is filled into a cavity via a gate while passing through a sprue and runner provided in the die. The resin molded material is removed from the die once the resin filled into the cavity has solidified. Consequently, a molding article (here, an optical element) corresponding to the cavity shape can be obtained by detaching the resin portion filled into the cavity from the resin portion filled into the gate.
FIGS. 5 to 7, etc. in Patent Document 1 illustrate a technology for performing injection molding such that optical axial thicknesses equal at respective borders of the gate part and the flange part prior to the gate being detached as a technology for injection-molding the abovementioned optical element (for example, a long lens). According to Patent Document 1, it is possible to prevent from generating weld lines caused by air involution at the molded site of the flange part in the vicinity of the gate by uniformly forming the connecting piece between gate [6b] and cavity [6a]. 